The ability of a brittle or thermoset resin to absorb energy without catastrophic failure can be increased through flexibilizing or toughening. Flexibilizing and toughening may be accomplished by reacting or compounding the resin with an elastomer thereby enhancing the resin system's ability to resist mechanical and thermal stress. Such elastomers are known and include reactive liquid polymers such as dicarboxyl-terminated polymers as exemplified by U.S. Pat. No. 3,285,949, and diamine-terminated polymers, as disclosed in U.S. Pat. No. 3,823,107. It is also known that liquid dicarboxyl-terminated polymers have the advantage of a material which is pourable and castable at room temperature and because of the reactive functional chain ends it can be further reacted at elevated temperature by the addition of polyamine compounds, to form the liquid diamine-terminated polymers. Such liquid elastomers have found a wide variety of utility, but are particularly useful as toughening agents in sealant, caulk, adhesive, and potting epoxy resin systems. In fact, heretofore it was believed that difunctionality of these polymers was essential to achieving desirable physical properties of the resins incorporating the elastomers. This thinking is typified by U.S. Pat. Nos. 3,823,107, 3,285,949, and the article "The Chemistry of Rubber Toughened Epoxy Resins I." by A. R. Siebert and C. K. Riew. Improvements in such properties as the impact strength and crack resistance of resins incorporating amine and carboxyl-terminated polymers have been attributed to their difunctional character. The terminal amine or carboxyl functionality at or close to two was believed essential to obtaining maximum toughness and therefore polymers having lesser functionality were thought to be unsuitable.